There are two types of organic EL display devices, a passive (a simple matrix) and an active (an active matrix), and development of both is being enthusiastically performed. Furthermore, organic materials are divided into low molecular weight (monomer) organic EL materials and high molecular weight (polymer) organic EL materials. Both are being vigorously researched, where a film of low molecular weight organic EL material is mainly formed by evaporation, while a film of high polymer organic EL material is mainly formed by application.
A drawback with organic EL devices is that they are difficult to drive using simple two-terminal schemes because of their lack of memory. The rise and decay time of an organic EL device is very fast and it does not have intrinsic memory. To overcome this problem, thin-film-transistor (TFT) circuits have been developed to drive organic EL devices. As illustrated in FIG. 1, these circuits include four or more TFTs, a storage capacitor and an organic EL pad arranged on a substrate. The storage capacitor enables the excitation power to an addressed EL element to stay on once it is selected.
While successfully overcoming the above-mentioned problem, new problems in manufacturing are created. The storage capacitor process and deposition are very complicated and difficult to achieve in a fabrication process. The TFTs fabrication requires several mask steps whose difficulty and cost increase dynamically as the display size increases. Plus if the substrate is plastic an expensive laser annealing process is used in fabrication of the TFT.